The present invention relates to a writing head in which plural arrays of writing electrode are formed on a flexible substrate for forming an electrostatic latent image on a latent the image carrier by applying a writing voltage to the electrode from a head driver. The present invention also relates to an image forming apparatus provided with the writing head.
In an image forming apparatus such as an electrostatic copying machine, a printer, the surface of a photosensitive member is uniformly charged by a charging device, and by exposing light from an exposing device such as a laser or an LED, an electrostatic latent image is formed in the surface of the photosensitive member. Thereafter, through the development of the electrostatic latent image in the surface of the photosensitive member, a developer image is formed on the surface of the photosensitive member. This developer image is transferred to a recording medium such as paper by a transferring device to record an image.
Such an image forming apparatus is likely to be bulky and has a complicated configuration since the exposing device for writing electrostatic latent images must be provided with a light generator.
Hence, there is proposed a head device for writing an electrostatic latent image on an image carrier with voltage applied through electrodes to omit the use of the light generator to downsize or simplify the image forming apparatus.
FIG. 1 shows one example of such a writing head. The writing head 3 is comprised of: a flexible substrate 3a; a plurality of strip-shaped wiring patterns 3c arrayed on the substrate 3a in a widthwise direction of a latent the image carrier 2 (described later); and writing electrodes 3b provided at the respective ends of the wiring patterns 3c so as to project toward the latent the image carrier 2.
The writing head 3 is manufactured by the following manner. Conductive members to be electrodes 3b such as cupper are first bonded on an insulative member to be the substrate 3a. Next, photoresist is coated on the conductive members. Next, a mask pattern corresponding to the wiring patterns 3c is laminated on the photoresist and an exposing process is performed.
Japanese Patent Publication No. 2002-172813A discloses a writing head (a first related art) in which writing electrodes are arranged in the widthwise direction of the latent the image carrier to form an electrode array, and two electrode arrays are arranged in a rotating direction of the latent the image carrier. Head drivers are disposed at one side or both sides of the two electrode arrays in the rotating direction of the latent the image carrier.
Japanese Patent Publication No. 2002-113897A discloses a writing head (a second related art) in which writing electrodes are pressed against a latent the image carrier with a pressing member or an urging member to establish a large nip width with a weak load.
In the first related art, since there is no wiring pattern at a portion on the substrate between the two electrode arrays, the stiffness of the portion becomes remarkably weaker than any other portions. In a case where stress is concentrated to the weak portion, deformation or creasing of the writing head may be occurred. In such a case, the two electrode arrays are hardly to be abutted against the latent the image carrier uniformly. As a result, electrostatic latent images cannot be correctly formed on the latent the image carrier, causing the deterioration of print quality. Moreover, another problem takes place that the creasing and bending of the writing head act to fluctuate the spacing between the two arrays of writing electrode, and thus giving horizontal streaks in the final image due to pitch fluctuation in the electrostatic latent image.
In the second related art, it cannot sufficiently prevent the bending and creasing of the writing head because the writing head is pressed with an extremely weak load.
Japanese Patent Publication No. 2002-178554A discloses a writing head (a third related art) in which a plurality of writing electrodes are arranged on a flexible base member in the widthwise direction of the latent the image carrier to form an electrode array. An electrostatic latent image is formed by charging the surface of the charge injection layer with predetermined voltage through the writing electrodes, in accordance with an input signal of image information. The writing electrodes are weakly abutted against the surface of a charge injection layer of the latent the image carrier with the flexibility of the base member, so that the abutment condition of the electrodes can be stabilized to securely perform the writing operation of the latent image.
In the third related art, two electrode arrays are arranged in the rotating direction of the latent the image carrier. The writing electrodes of the two electrode arrays are arranged in a zigzag manner such that one electrode in the first array is situated between adjacent electrodes in the second array. Since the electrodes are partially overlapped relative to the rotating direction of the latent the image carrier, non-chargeable region will not be formed on the surface of the latent the image carrier. That is the entire surface of the latent the image carrier is made chargeable.
In this case, the writing electrodes in the array closer to the fixed end of the flexible base member is abutted against the latent the image carrier more strongly than writing electrodes in the array closer to the free end of the flexible base member. Since the stiffness of the portion of the base member between the two electrode arrays is small, an elastic force for separating the electrode array closer to the free end of the base member from the latent the image carrier may act through the electrode army closer to the fixed end of the base member.
In such a case, the contact resistances of the two electrode arrays with respect to the latent the image carrier are made different from each other. In a case where the writing operation is performed by applying the same voltage to the two electrode arrays having different contact resistances, the latent image is deteriorated.